I. Field of the Invention
This invention relates to an improved Fischer-Tropsch process for the production of hydrocarbons, especially C.sub.10.sup.+ distillate fuels, and other valuable products. In particular, it relates to a process for the start-up of a Fischer-Tropsch reactor.
II. Background
A need exists for the creation, development, and improvement of processes useful for the conversion of synthesis gases to hydrocarbons, especially to high quality transportation fuels. Fischer-Tropsch synthesis, a process useful for the synthesis of hydrocarbons from carbon monoxide and hydrogen is well known in the technical and patent literature. The reaction is carried out catalytically by contact of the hydrogen and carbon monoxide with the catalyst, the reaction being accompanied by a large heat release. When this reaction is carried out in fixed-bed reactors, this high heat of reaction results in an increase in the temperature of the catalyst bed above that of the surrounding ambient. This temperatue rise is related to the amount of the occurring reaction and is the driving force for the transfer of heat from the catalyst bed to the surrounding reactor environment. Excessive temperature rises must be avoided in that they can lead to inferior product distribution, and possible temperature runaway which can damage the catalyst.
The Fischer-Tropsch hydrocarbon synthesis reaction is generally carried out in a fixed bed reactor, synthesis gas being fed via an inlet into direct contact with the catalyst while heat is removed from the catalyst bed via heat exchange between the partitioning wall which contains the catalyst and a heat exchange medium, e.g., water. It is advantageous during normal operation to maintain as high a temperature gradient as possible between the bed of catalyst in contact with the partitioning wall and the heat exchange medium used to remove heat from the partitioning wall because this provides the highest space-time-yield (defined as moles of carbon monoxide consumed/volume catalyst-time) with a minimum of reactor volume. The optimum temperature gradient must be one wherein the catalyst produces a product having the desired spectrum of hydrocarbons, while the catalyst bed remains thermally stable.
The temperature gradient between the catalyst bed and heat exchange medium depends upon the volumetric heat release. The temperature gradient also depends upon the space-time-yield, the size (diameter) of the reactor tube, and the heat transfer properties of the bed. From an economic point of view it is desirable to operate the reactor at the highest possible space-time-yield consistent with producing the desired product slate. Though simply stated, this objective is not so easily met in the start-up of a Fischer-Tropsch reactor. In general, when utilizing a 1/2 inch ID six foot long reactor, it requires from about 8 to 18 days to bring a Fischer-Tropsch reactor "on-stream," or to the point of "line out," i.e., the end of the start-up period, or point where the reactor begins and continues to produce the desired product slate at something approximating the desired space-time-yield. A larger reactor will require a longer period for start-up. Until the reactor lines out the selectivity and yield of product suffer. The productivity of the reactor is very low. The selectivity is less than optimum and the yield is considerably lower than achieved after line-out of the reactor. On top of this, the start-up period requires careful control with highly skilled operating manpower to avoid far more serious, and possibly catastrophic economic losses due to temperature runaway.
The productivity of any given reactor in conducting a Fischer-Tropsch reaction is limited by the amount of heat than can be put into the catalyst bed, and the amount of heat that can be removed from the bed within a given time period. Any change which results in an increase in the reaction rate, or decrease in the heat removal rate will result in an increase in the catalyst bed temperature. A temperature increase will cause a faster reaction rate, and this in turn will produce more heat and a higher temperature. On top of this, in any given catalyst bed the temperature profile across the bed is never entirely uniform. In any given catalyst bed there is invariably a maximum temperature or hot spot at which the local rate of reaction is the greatest, and in Fischer-Tropsch reactions the hot spot is not necessarily at the interface at which the synthesis gas first contacts the catalyst. The hot spot is the most sensitive portion of the reactor with respect to any perturbation in the process variables or heat removal rates of the system. Should the temperature become too highly excessive methane and carbon dioxide will be produced, and should more heat be produced than can be removed by the heat exchange medium for any sustained period the temperature may increase uncontrollably until all of the feed is consumed to produce principally methane and carbon dioxide, or the catalyst deactivated due to the excessive temperature, or both. The extreme temperature of a runaway can destroy the integrity and usefulness of the catalyst located in a hot spot zone and, albeit eventually this portion of a bed may cool due to catalyst inactivation, the excessive heat may cause temperature runaways in other portions of the bed until the entire catalyst bed is completely deactivated. Temperature runaways can cause metallurgical failure of the reactor vessel due to the high temperature and pressure.
In practice, start-up is the most difficult aspect of the Fischer-Tropsch reactor operation. Catalyst beds can readily heat up in localized areas when feed gas is first introduced. During start-up, transient instabilities exist due to the dynamic temperature profiles within and across the catalyst bed. For a given reactor operating with a stable temperature profile, the maximum space-time-yield is controlled by the temperature, pressure, and gas rate since these factors control the rate of carbon monoxide conversion. Under steady state conditions and constant space-time-yield the conventional wisdom is that the ratio of hydrogen:carbon monoxide in the feed gas will not affect the thermal stability of the reactor. Thus, a stable temperature profile is independent of the hydrogen:carbon monoxide feed ratio. The maximum space-time-yield, or volumetric heat release at which a reactor can operate depends entirely upon the heat removal characteristics of the reactor system.
In the start-up of a reactor, if a localized hot spot generates more heat than can be removed, a temperature runaway can occur. With the ratio of hydrogen:carbon monoxide fixed at the desired line-out value, one might therefore raise either the temperature and/or pressure in any combination to finally achieve the desired space-time-yield. However, in practice, the temperature profiles formed when pressure is increased as the last step are extremely unstable and in most cases will lead to a temperature runaway. Consequently, the accepted procedure is to first increase the pressure up to the value desired for normal operation while keeping the temperature at a low value. This minimizes the chances of a thermal runaway since the space-time-yield is kept very low. Once the operating pressure is obtained, the temperature is slowly increased in order to increase the space-time-yield. The procedure, while workable, requires considerable time to bring even a small reactor on-stream, and the reactor lined out to produce the desired product slate at maximum space-time-yield. During the start-up period the productivity, selectivity, and yield are abominable, and careful control by skilled operators is required to avoid temperature runaway.